Re: g77 & RAM access

[Mike Stump <mstump at apple dot com>]

On Wednesday, January 22, 2003, at 03:26 AM, Anuchit Aromsawa wrote:
> What is the maximum physical memory (RAM) on PC
> that g77 (or other fortran) can adrress?
With most memory models on most machines, one is limited to 2^n bytes 
in a single process, where n is either 32 or 64.  4GB for 32-bit 
systems, and a rather larger amount for 64 bit systems.  I don't give a 
specific number for 64 bit systems, as you asked about RAM, and I 
suspect few 64 bit systems actually support 18446744073709551616 bytes 
of ram, anyway, I suspect you don't have $800G lying around to populate 
your machine with RAM, even if it supported it.

These numbers are further discounted based upon language, runtime 
environment, OS and possibly  other factors.  For example, with g77 on 
Cygwin on Win 98, I'd be surprised if you got anywhere near 4GB.  900MB 
used to be the upper limit, before that, it used to be smaller.

Now, that is for normal memory models.  It is also possible on some 
machines (Windows) to use OS routines to address more than 4GB of RAM, 
but, to do so not at the same time, you map in different parts of RAM 
at different times.  The user code has to manage this and page it into 
the virtual address space on demand, just before use.  This also 
complicates the answer to your question greatly.  There isn't a reason 
why one couldn't use the moral equivalent of the same trick under 
Linux, so then the limit would be the amount of ram you can fit inside 
the box.  From, http://www.supermicro.com/iebc/Supermicro.htm, we see 
that 32GB is fairly easy to come by, so certainly at a minimum, 32GB 
would be one answer, however, if you have money, you can buy almost any 
amount you want, and if you want to play software tricks, there really 
is no limit, other than your wallet.

AMD's new processor is limited to right around 2^40, or right around 
1TB, so if you wanted, you could probably have someone build you a 
Linux box that had 3/4TB of user land RAM directly addressable per 
process, without any tricks.

Hopefully I've given you a smattering of numbers that you can select 
one you like, as they are all different.  If you don't like those, I'm 
sure others can give you even more numbers, though, this list isn't 
quite the right place for a wondering discussion.

The other answer, is to write a program that calls malloc, and run it 
on your PC, and just print the largest number that you can call malloc 
with that returns non-zero.  :-)